Machines for making and dispensing hot beverages provided with devices for heating water are known, generally defined as boilers or kettles; such devices, usually supplied by electric current, are able to heat the water contained in a tank, by means of a heating element.
The heating element, usually made of a resistive material, is permanently dipped in the water contained in the tank; a potential difference is applied at the ends of the heating element, then an electric current is generated that, due to Joule effect, dissipates energy in form of heat by heating the water. In practice, it is the same technique used for heating water inside a conventional water-boiler.
It is then necessary to maintain the water contained in the heating device at a desired temperature, also when the machine is not operating, so that to assure a dispensing of the beverage at the desired temperature without long waiting times. Then, if the machine is not operative for a long time, there will be a great energy consumption for maintaining the water at high temperatures (usually higher than 85° C.).
For example, in case of boilers used for the preparation of beverages by means of soluble substances, at the end of every dispensing, the level of hot water contained in the boiler is reduced and water at ambient temperature is entered in order to fill again the boiler. Then, there is a reducing in the whole temperature of water and, to assure that the following dispensing of a beverage will happen at the desired temperature, it is necessary a more or less long waiting time to heat again the water; the waiting time will depend on the amount of hot water fed in one or more preceding dispensing.
In addition to the temperature, an important specification to be fulfilled is the flow rate of the fed hot water, above all depending from the beverage type to be made; for example, in case of beverage produced by means of soluble substances, a considerable flow rate of hot water is required (at least 10 cc/s); it is not simple to satisfy this specification, in fact with a high flow rate of fed hot water there will be a quick decrease of temperature of water contained in the tank, resulting in long waiting times for a following dispensing or else obtaining a beverage whose soluble substance could give some lumps.
The main problems related to water heating, in machines for making and dispensing beverages, are due to thermal inertia with which a determined water mass is heated; in order to have a quick water heating the phenomenon of electromagnetic induction can be used for generating eddy currents inside an electrically conductive element in contact with the water to be heated; the eddy currents dissipate energy, because of Joule effect, in form of warmth heating the conductive element and consequently the water contacting therewith.
It is known that electromagnetic induction heaters have the quality of heating rapidly the water flowing inside a metal pipe in which the eddy currents are induced.
U.S. Pat. No. 5,262,621 in the name of Industrial Technology Research Institute, shows an apparatus for heating the water flowing inside a metal pipe in which a winding of electromagnetic induction is wound. The winding is supplied with an AC voltage, hence, inside the pipe, eddy currents are generated which heat the pipe thereby increasing the temperature of water flowing in its inside. The water passes through the pipe by gravity, therefore no dispensing of hot water with high flow rate is arranged. Also in DE10350064 is shown a device for heating water in a coffee machine in which the winding of electromagnetic induction is wound around a chromium-plated steel pipe having a thin wall.
Anyway, the afore said heaters have a problem: inside the heating pipe the limescale contained in the water will deposit because of heating; then, inside the heating pipe, an inner coating of limescale will form that, at the beginning, limits the heat conduction from the pipe towards the water and, successively, will be able to block the inside of the heating pipe with the relative damage of the device or the whole machine. Therefore, in order to avoid damages of the machine or the replacement of the heater, a cleaning cycle will be periodically carried out, by means of descaling substances, to remove the limescale from the heating pipe.
For example EP2044869 shows an induction heater provided with an ultrasound generator; the heater comprises a cylindrical casing made of ferromagnetic material on which a winding of electromagnetic induction is wound; inside the ferromagnetic casing there is a flow path for the water to be heated and an ultrasound generator is integrated at one end of said casing; in this way the limescale formation inside the heater is hindered by the ultrasonic vibrations made by the ultrasound generator.
Then, the above mentioned heaters have the problem of needing an accurate maintenance of the heating duct. In case in which the heating duct is blocked, the replacement of these heaters is a complex and expensive operation.